1. Field of the Invention
The present invention relates generally to improved interbody spinal fusion implants for the immobilization of adjacent vertebral bodies and to a method for deployment thereof. In particular, the invention relates to interbody spinal fusion implants and methods for deployment thereof that significantly preserve the structural support of the dense endplate and subchondral bone regions of the adjacent vertebral bodies while also penetrating those endplates so as to access the vascular subchondral bone of those vertebral bodies for the purpose of achieving interbody spinal fusion at least in part through the implants themselves.
2. Description of the Prior Art
Surgical interbody spinal fusion refers to the method of achieving a bridge of bone tissue in continuity between adjacent vertebral bodies and across the disc space to thereby substantially eliminate relative motion between these adjacent vertebral bodies. The term “disc space” refers to the space between adjacent vertebral bodies normally occupied by a spinal disc. The spinal disc that normally resides between the adjacent vertebral bodies maintains the spacing between those vertebral bodies and, in a healthy spine, allows for the normal relative motion between the vertebral bodies.
Numerous implants to facilitate fusion have been described by Cloward, Brantigan, Michelson, and others, and are known to those skilled in the art. Such fusions have also been achieved with the use of bone grafts placed between the vertebral bodies, such as taught and practiced by Dr. Cloward. Generally, cylindrical implants, which may be threaded, offer the advantage of conforming to an easily prepared recipient bore spanning the disc space and penetrating into each of the adjacent vertebral bodies. Such a bore may be created by use of a drill. Drilling of the bore, however, removes a portion of the endplates and of the subchondral bone.
Human vertebral bodies have a hard outer shell of compacted, dense cancellous bone (sometimes referred to as the cortex) and a relatively softer, inner mass of cancellous bone. Just below the cortex adjacent the disc is a region of bone referred to herein as the “subchondral zone”. The outer shell of compact bone (the boney endplate) adjacent to the spinal disc and the underlying subchondral zone are together herein referred to as the boney “end plate region” and, for the purposes of this application, is hereby so defined to avoid ambiguity. The endplate region constitutes the densest bone available to support the fusion implant over its length, and removal of this endplate region by the practice of creating a bore into the vertebral bodies results in the implant coming to rest on the softer and less dense cancellous bone that lies beneath the endplate deeper within the vertebral body.
Other spinal fusion implants are known that incorporate a modified cylindrical or a tapered cylindrical shape that also require the use of a drill to create a bore across the disc space and also result in the removal of a portion of the endplate. Inasmuch as the upper and lower vertebral bodies—contacting surfaces of these types of implants are arc-shaped, absent arching the recipient bed in the vertebral body by drilling, it would not be possible to gain the contact between the vertebral bodies and implant needed to achieve fusion. Such arching of the vertebral bodies to receive the implant results in the removal of the endplate.
Non-cylindrical implants that are pushed into the disc space after a discectomy are also known in the art. While these push-in implants do have the advantage of supporting the adjacent vertebral bodies by contacting a substantial portion of the vertebral endplates, they do not offer the advantages associated with threaded cylindrical implants that are screwed into a bore in the adjacent vertebral bodies to more securely hold these implants in their final fully seated positions. Further, unless the endplate is at least partially decorticated, i.e. worked upon to access the vascularity deep to the outer most aspect of the endplate itself, fusion will not occur.
Non-cylindrical spinal fusion implants that are inserted between the endplates of adjacent vertebral bodies and then rotated 90 degrees into place are also known. However, their cross-sectional configuration causes either unwanted over-distraction of the vertebral bodies as they are rotated or under-distraction between the adjacent vertebral bodies once rotated. For example, an implant having an approximately square or rectangular cross-section when rotated in either a clockwise or counterclockwise direction will result in a maximum distraction of the disc space when the diagonal of the implant is at a right angle (90 degrees) to the adjacent vertebral endplates. This amount of distraction is greater than that achieved by the implant when either of its opposed sides are in contact with the adjacent vertebral bodies. If the space between the adjacent vertebral bodies is too small or the amount of attempted distraction too great, rotation of the implant will either not be possible or the vertebral bodies will be broken. If the space between the adjacent vertebral bodies is sufficiently large to permit rotation of such an implant, then when the implant is rotated to its final position with its opposed sides in contact with the adjacent vertebral bodies, insufficient distraction will be achieved between the vertebral bodies as the opposed sides will have a lesser height between them than the diagonal which rotated through that same space. It should be noted that distraction within the elastic range of deformation is highly desirable because it secures the implant, allows the implant to stabilize the adjacent vertebral bodies relative to each other, and provides the most space for the neural elements both passing through and exiting through those vertebral segments.
Therefore, there exists a need for a spinal fusion implant that permits the endplate region of the adjacent vertebral bodies to be substantially preserved while nevertheless accessing the underlying bone vascularity and which implant can be rotated 90 degrees within the disc space to achieve the optimal distraction in the range of elastic deformation and short of plastic deformation and tissue failure.